American Nurseryman, September 2010

There are crabapples with good scab resistance in so many different avors that indeed it is an embarrass-ment that those in the Northeast and Midwest still plant Royalty, Radiant and Hopa. We say, No Hopa!Crabmandu predicts!One thing we learned early on is that predicting how bad scab will be in an upcoming season leaves a sour taste when the future becomes the present. We remember being asked in the winter of 2000 to predict if scab would be bad in the coming season. Thinking of the dis- ease triangle, we considered that overall scab incidence in 1999 in Crablandia was the lowest in the past seven years. We know that scab gets started each spring from V. inaequalis in fallen, scab-by leaves from the previous years. Fewer scabby leaves in 1999 equals less fungal inoculum for 2000, right? Our con dent prediction for 2000: a low scab year. Not. When we retrospectively looked at our data from 1992 to 2006, we discovered that actually 2000 was the highest scab year of all in Crab- landia. Apparently, the relatively lesser amount of overwintering inoculum was not the limiting factor for how bad scab would be in 2000. As it turned out, the key was the extremely wet spring of 2000. It turned out that even though there was less inoculum, there was still enough overwintering V. inaeqaulis for a banner scab year since the environ- ment was right for scab development. Spring moisture and its role in in-fection periods for scab is what drives how bad scab will be in a given year. The bad news is that our ability to pre- dict scab for an upcoming season is as good as our ability to predict the weath- er in the upcoming season.Scab is not just for foliageTypically when we think of scab, it is of the discolored foliage and leaf drop that diminishes the aesthetics of the tree. And when many think of the orna- mental features of crabapples, the most likely feature is the marvelous owers. Yet fruits actually have a longer and richer ornamental life than owers. Any individual crabapple tree has maybe 10 to 14 days of prime ower show, while fruits can be ornamentally effective for months. With Red Jewel, for example, this can extend up to six months. By the same token, scab can mar fruits, taking away their namesake contribution to a particular cultivar. A classic example is Weeping Candied Apple, which, as you might imagine, has very shiny, bright red candy-apple fruits. Except almost every year in Ohio (though not elsewhere; see below) they turn into grayish, shriveled lumps due to fruit scab. Weeping Candied Apple does not really have heavy leaf scab, but the amount of fruit scab ruins its great- est ornamental potential in this partic- ular location. Scab never sleepsOne of the realities of nature is ex-pressed in the saying Naturam expel- las fuca, tamen usque recurrat. Trans- lation: You can drive out nature with a pitchfork, but she always returns. Plant breeding and selection, and propaga- tion of superior taxa are among our most storied horticultural pitchforks. Certainly genetic resistance to apple scab is one of the features we look for in such breeding and selection programs. However, nature is also out there, conspiring to alter our best efforts. From her point of view, the V. inae-qualis fungal pathogen is every bit as deserving to thrive as the beauty of the genus Malus , and so she performs her own experiments. And so, through ge- netic recombination and mutations, the fungus has its own breeding pro- gram. New strains of the fungus appear. Thus, Indian Magic and Harvest Gold, which decades ago were evaluated as having good scab resistance in Ohio tri- als, are now seen to have a signi cant scab problem, presumably due to new strains of the fungus that have evolved. On a more recent note, Prairi re in our Ohio Crablandia plots never ex- hibited scab for several decadesthat is, until several years ago. Scab on Prai- ri re is still minor in Ohio, but only time will tell how it will hold up over the Why Plant Evaluations Matter Continued from page 11 A fresh reblight strike, or infection, causes the current seasons new growth on crabapple to collapse, or ag.The process of plant disease always involves the interplay of a susceptible host, a virulent pathogen and an environment conducive to disease . All are needed for disease to occur. 12 AMERICAN NURSERYMAN SEPTEMBER 2010 www.amerinursery.com A N _ 0 9 1 0 _ p g s 1 -2 1 . i n d d 1 2 AN_0910_pgs1-21.indd 12 9 / 8 / 1 0 1 2 : 1 2 : 4 0 P M 9/8/10 12:12:40 PM

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